Editing Declination (section)

== Effects of precession ==
[[File:Ra and dec on celestial sphere.png|thumb|300px|[[Right ascension]] (blue) and '''declination''' (green) as seen from outside the [[celestial sphere]].]]

{{Main|Axial precession}}

The Earth's axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as [[Axial precession|precession]], causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, [[Equatorial coordinate system|equatorial coordinates]] (including declination) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an [[Epoch (astronomy)|epoch]]. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.<ref>Moulton (1918), pp. 92–95.</ref>

The currently used standard epoch is [[J2000.0]], which is January 1, 2000 at 12:00 [[Terrestrial Time|TT]]. The prefix "J" indicates that it is a [[Julian epoch]]. Prior to J2000.0, astronomers used the successive [[Epoch (astronomy)#Besselian epoch|Besselian Epochs]] B1875.0, B1900.0, and B1950.0.<ref>
see, for instance,
{{cite book
 | last = U.S. Naval Observatory Nautical Almanac Office
 | first = Nautical Almanac Office
 | author2 = U.K. Hydrographic Office, H.M. Nautical Almanac Office
 | title = The Astronomical Almanac for the Year 2010
 | publisher = U.S. Govt. Printing Office
 | date = 2008
 | page = B2
 | chapter = Time Scales and Coordinate Systems, 2010
 }}</ref>